Why does this variable need to be static?

Question

class armon
{
    static const int maxSize=10;    

    int array[maxSize];

    int count=0;

    int* topOfStack=array;
}

Why does maxSize need to be static for it to be used inside array?

Answer 1

It doesn't have to be static, but it must be a constant expression.

C++ Standard § 8.3.4 [dcl.array] (emphasis mine) :

If the constant-expression (5.19) is present, it shall be a converted constant expression of type std::size_t and its value shall be greater than zero

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That is, the following is also valid :

constexpr std::size_t Size()  { return 10; }; 

struct Y
{
    int array[Size()];
};

---

Note:

Since the compiler needs to know the size of the class, you cannot do this :

struct Y
{
    const int size = 10;
    int array[size];
};

Possibly making different instances of Y having different sizes.

Also note that in this context, int array[size] is not a constant expression, because it makes use of this, see the C++ standard section § 5.19 [expr.const] :

A conditional-expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine (1.9), would evaluate one of the following expressions:

— this (5.1.1), except in a constexpr function or a constexpr constructor that is being evaluated as part of e;

(the evaluation of size is really this->size)